docs: def _null_ac_sia(transition, direction, alpha=0.0):
      """Return an |AcSystemIrreducibilityAnalysis| with zero |big_alpha| and
      empty accounts.
      """
      return AcSystemIrreducibilityAnalysis(
          transition=transition,
          direction=direction,
          alpha=alpha,
          account=(),
          partitioned_account=()
      )